The aim of this study is to evaluate the antidiabetic effect of the water extract of Aurantii fructus immaturus (WAF), in diabetic models using enzyme, cells and mice, and to suggest a putative mechanism explaining its antidiabetic effect. In an enzyme model using the enzyme α-glucosidase, WAF had no significant effect on α-glucosidase, as compared with acarbose, an antidiabetic drug. Nonetheless, WAF was capable of reducing the blood glucose levels during oral sucrose tolerance test and oral glucose tolerance test, implying that there would be other antidiabetic pathways in no relation to inhibition of α-glucosidase. In cell models using RIN-m5f β-cells and L6 myotubes, WAF, at its non-cytotoxic doses, augmented the secretion of insulin in RIN-m5f β-cells stimulated with 5 mM glucose. In addition, it enhanced the cellular uptake of glucose in L6 myotubes stimulated with deprivation of glucose for 12 h. Therefore, it is most likely that WAF may exert its antidiabetic effects, at least in part, by enhancing insulin secretion and glucose uptake. Meanwhile, in diabetic mice induced with peritoneal injection of streptozotocin (STZ), WAF significantly improved fast blood glucose levels, glycosylated hemoglobin levels, body weight loose, blood pressure, and diabetic adverse effects on functions of the kidney and the liver. Taken together, the water extract of Aurantii fructus immaturus may ameliorate diabetes in mice injected with STZ, at least in part, by enhancing insulin secretion and glucose uptake.